The present invention relates generally to radar tracking systems, and more specifically the invention pertains to a signal processor for use with the multioctave ultra-wideband radar tracking systems.
Electronically scanned phased arrays are replacing mechanically scanned reflectors for modern radar applications. New design problems and new beam agility requirements arise due to the modern target threat. The trend toward increasing waveform bandwidth in radar further complicates this radar design problem. It should be noted that phase shift techniques for beam steering are applicable to phased array radars employing waveform modulation bandwidths of less than one percent. For larger bandwidth waveforms, true time delay techniques are used. Broadband systems which require many beam positions will require a large number of time delay lines and microwave switches. Also, the corporate feed loss of such a beamforming system will be excessive, and will vary with frequency, thus causing signal distortion and reduced radar sensitivity. Therefore, for ultra wideband radar waveforms, with an octave bandwidth or more, true time delay techniques are also inadequate.
The technique of electronically steering a narrowband radar beam by adjusting the phase of adjacent radiating elements is described in the text "Introduction to Radar Systems" by M. Skolnik, the disclosure of which is incorporated herein by reference. A change in relative phase between adjacent elements may be obtained by a change in frequency. This principle can be used to scan a beam from an array if the phase shifters are frequency-dependent. A frequency-scanned antenna might be represented by a series-fed array with fixed lengths of transmission line connecting the elements. The total phase delay through a fixed length l of transmission line is 2.pi.fl/c, and thus is a function of the frequency f. The lines connecting adjacent elements of the series-fed frequency-scanned array are of equal length and chosen so that the phase at each element is the same when the frequency is the center frequency f.sub.o. When the frequency is exactly f.sub.o the beam points straight ahead. As the frequency is increased above f.sub.o, the phase through each length of transmission line increases and the beam rotates to one side. At frequencies below f.sub.o the beam moves in the opposite direction.
The technique of frequency scanning to electronically steer a radar beam is particularly important in ultra-wideband radar systems. Examples of the use of frequency scanning radar systems are discussed in the following U.S. Patents, the disclosures of which are specifically incorporated herein by reference:
U.S. Pat. No. 4,912,474 issued to Paturel;
U.S. Pat. No. 4,868,574 issued to Raaab;
U.S. Pat. No. 4,827,229 issued to Sabet-Peyman;
U.S. Pat. No. 4,516,131 issued to Bayha;
U.S. Pat. No. 4,276,551 issued to Williams et al;
U.S. Pat. No. 3,434,139 issued to Alego;
U.S. Pat. No. 4,160,975 issued to Steudel;
U.S. Pat. No. 4,683,474 issued to Randing; and
U.S. Pat. No. 4,861,158 issued to Breen.
Both frequency and phase scanned antennas are well known in the art. Frequency scanned antennas have the advantages of simplicity and low cost. The patent to Steudel teaches a correction circuit or using the sum and difference signals in wideband antenna system to increase azimuth and elevation accuracy. The patent to Randing teaches a ground base sensor comprising a plurality of unconnected sub-arrays, a wideband receiver matched filter bank, envelope bank selectors, a summing network and a target detector. The patent to Breen teaches a device for performing a doppler shift measurement with a chirp measurement with a chirp frequency laser signal.
Conventional radar technology implies systems utilizing waveforms with modulation bandwidths of up to 2%, while modern wideband radar systems utilize bandwidths of less than 25% Present research and development efforts involve expanding bandwidths into ultra-widebands as follows.
Typical radar systems transmit waveforms with frequencies selected from a range of between 300 MH.sub.z to 40 GH.sub.z. In most case the radar systems include a single band device. That is, the system operates on only one frequency band. Thus, two (or more) array apertures are required in order to radiate and receive multiband radar waveforms. In the past, this has caused the multi-frequency systems to have multiple apertures with the attendant increase in cost, weight, size and the like. Thus, these systems have been disadvantageous for utilization in many applications.
The task of utilizing ultra-wideband radar frequency is alleviated, to some extent, by the system disclosed in the U.S. Pat. No. 4,689,627 issued to Lee et al., the disclosure of which is incorporated herein by reference.
The above-cited Lee et al. reference discloses an ultra-wideband radar system which can operate over approximately an octave bandwidth encompassing, for example, both S-band and C-band. The present invention can make use of the Lee et al. system to transmit a multioctave chirp waveform in a process which eliminates range-doppler ambiguities.
For multi-octave bandwidth radar systems employing frequency modulated waveforms, the present invention solves the problems associated with beamforming and beam agility. The present invention utilizes phased array antenna technology, receivers on each antenna element, and baseband frequency offset generation techniques, thus eliminating corporate feed losses and signal distortion. Also, the present invention provides for the generation of multiple beams, which is required for many modern radar applications.